1. Field of the Invention
This invention relates to a method for making a magnetic recording medium. This invention particularly relates to a method for making a magnetic recording medium, which comprises a non-magnetic substrate and a thin ferromagnetic metal film overlaid on the non-magnetic substrate and which exhibits improved resistance to corrosion.
2. Description of the Prior Art
As media for recording and reproducing magnetic information, coated types of magnetic recording media have heretofore been used widely. The coated types of magnetic recording media comprise a non-magnetic substrate and a magnetic layer overlaid on the non-magnetic substrate. The magnetic layer is formed by applying a magnetic coating composition, which contains magnetic grains and an organic binder, or the like, to the non-magnetic substrate and then drying it.
Nowadays there is a strong demand for magnetic recording media on which magnetic information can be recorded at high densities. This demand cannot be satisfied with the conventional coated types of magnetic recording media. For example, in a Hi-8 system, magnetic information is recorded at a high density with wavelengths shorter than 0.5.mu.m. For such a system, thin metal film types of magnetic recording media are substantially suitable and have been used in practice. The thin metal film types of magnetic recording media comprise a non-magnetic substrate and a thin ferromagnetic metal film overlaid on the non-magnetic substrate. The thin metal film types of magnetic recording media are advantageous in that they have a high magnetic energy level and the thickness of the magnetic layer can be kept thin.
However, the thin metal film types of magnetic recording media have the drawbacks in that their thin magnetic metal layers are easily corroded by water and oxygen.
Research and development of thin metal film types of magnetic recording media (tapes) are mainly directed to vapor deposition tapes, which have a magnetic layer formed with an oblique incidence vacuum evaporation process wherein a CoNi alloy is evaporated in oxygen. Originally developed vapor deposition tapes have the problems in that they rust when they are left to stand in the air, and their magnetic properties deteriorates to a half level with the passage of time of approximately one year. Therefore, the originally developed vapor deposition tapes are not satisfactory from the point of view of storage of magnetic information.
Various attempts have heretofore been made to eliminate the following problems with regard to corrosion of vapor deposition tapes:
1) Ambient moisture condenses and dries on thin ferromagnetic metal films, and the thin ferromagnetic metal films thereby rust. PA1 2) Magnetic properties of thin ferromagnetic metal films deteriorate (i.e. they are demagnetized) during their storage at a high temperature and high humidity. PA1 3) Surfaces of thin ferromagnetic metal films are caused to rust by sulfur dioxide, nitric oxide, or the like, which is present in the air. PA1 4) Surfaces of thin ferromagnetic metal films are caused to rust by salts, such as NaCl, which are contained in small droplets of brine, in areas near coasts. PA1 i) forming a thin ferromagnetic metal film on a non-magnetic substrate, said thin ferromagnetic metal film containing Co as a main constituent and containing oxygen in a proportion of at least 15 at%, and PA1 ii) thereafter exposing said thin ferromagnetic metal film to an ozone-containing atmosphere.
As for the problems in (1), (2), and (3), various techniques have been proposed, which can improve the corrosion resistance of thin ferromagnetic metal film to a practically acceptable level. However, as for corrosion with salts in (4), no suitable means has yet been found.
Specifically, a certain technique, which is suitable for eliminating one of the problems in (1), (2), and (3) and keeping the corrosion resistance of vapor deposition tapes high, is not necessarily advantageous for the elimination of all of the problems described above, or causes other new problems to occur. Therefore, the conventional techniques for keeping the corrosion resistance of vapor deposition tapes high are not suitable from the point of view of the whole characteristics of magnetic recording media.
For example, a method has been proposed wherein an inorganic protective layer constituted of SiO.sub.2, C, or TiO.sub.2 is overlaid on a thin ferromagnetic metal film. With the proposed method, in order for a necessary level of corrosion resistance to be obtained, the thickness of the protective layer must be at least 300.ANG.. Therefore, because of spacing loss, the reproduction output power decreases. The reproduction output power should be prevented from decreasing, in particular, in cases where magnetic information is recorded with short wavelengths which are ordinarily employed for magnetic recording media provided with thin metal films.
Also, methods have been proposed wherein thin ferromagnetic metal films are imparted with corrosion resistance through various types of post-treatment. For example, a method has been proposed wherein a magnetic recording medium is kept at a predetermined temperature and predetermined humidity. Also, a method has been proposed wherein a passivity film is formed by an electrochemical technique. Additionally, a method has been proposed wherein oxidization treatment is carried out on vapor deposition tape webs. With these proposed methods, the problems in (1) and (2) can be eliminated, and resistance to corrosion with moisture can thereby be improved. However, with the proposed methods, thin ferromagnetic metal films cannot be prevented from rusting when they are exposed to salts.
Additionally, novel methods for improving corrosion resistance of thin ferromagnetic metal films have been disclosed in, for example, Japanese Unexamined Patent Publication Nos. 58(1983)-26319, 58(1983)-26322, 58(1983)-60432, 59(1984)-63031, and 59(1984)-60738. With the disclosed methods, ozone is brought into contact with thin ferromagnetic metal films while they are being formed, or thin ferromagnetic metal films which have been formed are exposed to an ozone-containing atmosphere. The disclosed methods have larger effects of improving corrosion resistance of thin ferromagnetic metal films than the conventional methods.
However, with the disclosed methods, resistance of thin ferromagnetic metal films to corrosion with salts described above in (4) cannot be kept sufficiently high.